A background-art technique disclosed in Patent Literature 1 is provided as a background-art technique of this type. This background-art technique is provided in a hydraulic excavator having a upperstructure, and includes a hydraulic pump, a swing motor which is operated by pressure oil discharged from the hydraulic pump so as to drive the upperstructure, a parking brake which applies a braking force to the swing motor, a swing direction control valve which controls the flow of the pressure oil supplied from the hydraulic pump to the swing motor, and a swing operation tool which operates to change over the swing direction control valve. In addition, the background-art technique also includes a parking brake control unit including a control portion and a changeover valve for controlling the parking brake so as to apply a braking force to the swing motor when a predetermined delay time has passed since the swing operation tool is brought back to a neutral position from an operation position.
Patent Literature 2 also discloses a similar technique to that in the aforementioned Patent Literature 1. In Patent Literature 2, there are provided a brake release cylinder which applies a braking force to a hydraulic motor, a hydraulic operation valve which controls the flow of pressure oil supplied from a hydraulic pump to the hydraulic motor, and a pilot changeover valve which operates to change over the hydraulic operation valve, and there is also provided a pressure-compensated flow rate control valve including a choke for controlling the brake release cylinder so as to apply a braking force to the hydraulic motor when a predetermined delay time has passed since the pilot changeover valve is brought back to a neutral position from an operation position.
According to these background-art techniques disclosed in Patent Literatures 1 and 2, a braking force is applied to the swing motor or the hydraulic motor by the parking brake or the brake release cylinder when the predetermined delay time has passed since the swing direction control valve or the pilot changeover valve is brought back to the neutral position from the operation position, so that the upperstructure or the like can be braked and stopped while the wear damage of the parking brake or the brake release cylinder is suppressed.
A configuration disclosed in Patent Literature 3 fundamentally has a different technical idea from those in the aforementioned background-art techniques disclosed in Patent Literatures 1 and 2. In the configuration disclosed in Patent Literature 3, a connection/disconnection unit including a gate lever, a gate lever switch and a hydraulic lock valve for connecting/disconnecting a swing operation tool to/from a pilot pump is provided in a hydraulic excavator having a travel base and a upperstructure. As soon as a swing direction control valve and the pilot pump are disconnected by the connection/disconnection unit, a braking force is applied to a swing motor by a brake cylinder immediately in electrical response to the disconnection. In response to the stop of the swing motor, a swing drive for driving the upperstructure is stopped. The background-art technique disclosed in Patent Literature 3 is apt to cause undesired action as follows. That is, the travel base provided under the upperstructure may move due to the inertia of the upperstructure because the upperstructure is stopped without a delay time but immediately after the connection/disconnection unit is operated in the state where the upperstructure is swinging. In addition, due to the operation of stopping the upperstructure as described above, wear damage may occur easily in a parking brake which applies a braking force to the swing motor.